Electrolytic copper plating solution for filling for forming microwiring of copper for ulsi

ABSTRACT

An electrolytic copper plating solution for filling for forming microwiring for ULSI, is characterized in that it has a pH of from 1.8 to 3.0. The electrolytic copper plating solution preferably contains a saturated carboxylic acid having from 1 to 4 carbon atoms at a concentration from 0.01 to 2.0 mol/L.

This is a division of Ser. No. 13/378 529, filed Dec. 15, 2011, whichwas the national stage of International Application No.PCT/JP2010/060545, filed Jun. 22, 2010, which International Applicationwas not published in English.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrolytic copper plating solutionfor filling for forming microwiring of copper for ULSI.

2. Description of the Related Art

Electrolytic copper plating for filling ULSI microwiring is usuallycarried out with sulfuric acid-based strongly acidic plating solutions(pH of 1.2 or lower). Seed layers upon plating are sputtered copperfilms, of which thickness in trenches/vias have become very thin due tominiaturization of wiring. Oxidation of the outermost layer of the seedlayer is unavoidable because the outermost layer is atmosphericallyexposed prior to electrolytic plating. Accordingly, the oxidized partsof the seed layer are easily dissolved when it is immersed in a stronglyacidic electrolytic plating solution leading to formation of defects onthe thin seed layer. When a copper wiring layer is formed byelectrolytic copper plating thereafter, the copper plating isunsatisfactorily absent in spots. Particularly, the inner wall oftrenches/vias tends to have problematic voids.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electrolytic copperplating solution which can suppress, upon electrolytic copper plating ona copper seed layer during fabrication of ULSI copper microwiring(damascene copper wiring) having trends to further miniaturization,dissolution of the copper seed layer and accordingly can suppressoccurrence of voids on the inner wall of vias/trenches.

The present inventors have attempted to suppress dissolution of copperseed layers upon being immersed in the plating solution by usingcarboxylic acids and the like instead of sulfuric acid usually used forsulfuric acid-based strongly acidic copper plating solutions and tochange pH of the plating solution from conventional strong acid toaround weak acid. As a result, they have found that formation of voidson the side wall of trenches which may be generated with the sulfuricacid-based strongly acidic plating solutions can be avoided with thecarboxylic acid-based plating solution, thereby accomplished the presentinvention which solves the above-mentioned problems.

Thus, the present invention provides the followings:

-   -   (1) an electrolytic copper plating solution for filling for        forming microwiring for ULSI, characterized in that it has a pH        of 1.8 or higher and 3.0 or lower;    -   (2) the electrolytic copper plating solution for filling for        forming microwiring for ULSI according to the above (1), wherein        it has a pH of 2.0 or higher and 2.2 or lower;    -   (3) the electrolytic copper plating solution for filling for        forming microwiring for ULSI according to the above (1) or (2),        wherein it comprises a saturated carboxylic acid having 1 or        more and 4 or less carbon atoms at 0.01 mol/L or more and 2.0        mol/L or less;    -   (4) the electrolytic copper plating solution for ULSI        microwiring according to the above (3), wherein the carboxylic        acid is acetic acid;    -   (5) a method for electrolytic copper plating for ULSI        microwiring, characterized in that it uses the electrolytic        copper plating solution for filling for forming microwiring for        ULSI according to any of the above (1) to (4); and    -   (6) a ULSI microwiring substrate characterized in that a ULSI        microwiring is formed by the method for electrolytic copper        plating for ULSI microwiring according to the above (5).

In ULSI copper microwiring (damascene copper wiring) fabrication,formation of a copper wiring layer on a copper seed layer using theelectrolytic copper plating solution of the present invention cansuppress dissolution of the copper seed layer and accordingly formationof voids on the inner wall of vias/trenches.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional SEM photograph illustrating that voids werenot formed in the side wall portion of trenches using the method of thepresent invention.

FIG. 2 is a cross-sectional SEM photograph illustrating that voids wereformed in the side wall portion of trenches using the method ofComparative Example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The electrolytic copper plating solution for filling for formingmicrowiring for ULSI of the present invention has a pH of 1.8 or higherand 3.0 or lower. Conventional sulfuric acid-based copper platingsolutions are strongly acidic at pH 1.2 or lower. However, the presentplating solution can have a pH of 1.8 or higher and 3.0 or lower becausecarboxylic acid such as acetic acid is used instead of sulfuric acid. Byemploying pH of 1.8 or higher and 3.0 or lower, dissolution of copperseed layers can be suppressed and accordingly formation of voids on theinner wall of vias/trenches can be suppressed. pH is more preferably 2.0or higher and 2.2 or lower.

If pH is lower than 1.8, copper seed layers are easily dissolved due tolow pH and as a result, voids are also easily formed. If pH is higherthan 3.0, copper ions in the plating solution may be converted to oxidesor hydroxides to cause precipitations.

The carboxylic acid may be any carboxylic acid that can be dissolved inthe plating solution and adjust pH to the above range, and is preferablya saturated carboxylic acid having 1 or more and 4 or less carbon atomssuch as formic acid, acetic acid, propionic acid, butyric acid, oxalicacid and the like, with acetic acid being particularly preferable.

The plating solution preferably contains the carboxylic acid at 0.01 to2.0 mol/L and more preferably 0.2 to 1.0 mol/L. The concentration of thecarboxylic acid in the plating solution affects filling properties andpH such that the concentration of carboxylic acid at more than 2.0 mol/Lreduces pH of the plating solution to lower than 1.8, causing increasedformation of voids. When the concentration of carboxylic acid in theplating solution is less than 0.01 mmol/L, the plating solution has a pHof higher than 3.0, which may cause precipitations as described above.

The electrolytic copper plating solution of the present invention isaqueous and may comprise other components such as copper salts, chlorideions, trace additives and the like, which may be well known and are notparticularly limited.

Copper salts may include copper sulfate, copper nitrate, copper chlorideand the like, with copper sulfate being preferable. The plating solutionpreferably contains the copper salt at 0.05 to 1.5 mol/L and morepreferably 0.2 to 0.8 mol/L.

The plating solution preferably contains chloride ions at aconcentration of 0.3 to 3.0 mmol/L and more preferably 1.0 to 2.0mmol/L.

Trace additives may include promoters, inhibitors, leveling agents andthe like.

Promoters may include bis(3-sulfopropyl)-disulfide, disodium salt,3-mercaptopropanesulfonic acid and the like, which are preferablycontained in the plating solution at 1 to 30 mg/L.

Inhibitors may include polyethylene glycol, polypropylene glycol,copolymers thereof and the like, which are preferably contained in theplating solution at 10 to 500 mg/L.

Leveling agents may include Janus Green B, polyethyleneimine,polyvinylpyrrolidone and the like, which are preferably contained in theplating solution at 0.1 to 50 mg/L.

Plating operations using the electrolytic copper plating solution of thepresent invention are preferably carried out at a bath temperature of 20to 30° C. in view of stability of bath and deposition speed of copper.The cathode current density is preferably 0.1 to 5 A/dm2.

A material to be plated by electrolytic copper plating is a microwiringsubstrate such as semiconductor wafers and is preferably a siliconsubstrate having ULSI microwiring such as trenches/vias onto which acopper seed layer is provided.

The copper seed layer may be formed by well-known methods such assputtering and electroless plating.

The electrolytic copper plating solution of the present invention allowsto carry out plating without voids even when the copper seed layer intrenches/vias has a thickness of 2 nm or less.

Examples Example 1

A silicon substrate having ULSI microwiring was subjected toelectrolytic copper plating using the following plating solution. Thesilicon substrate, which is to be plated, has fine trench patterns (linewidth: 180 nm and depth: 500 nm) and a Cu seed layer is provided on itsoutermost surface by sputtering. The Cu seed layer had a thinnestthickness of 2 nm in trenches.

Composition of plating solution:

Copper (copper sulfate) 0.63 mol/L; Acetic acid 0.5 mol/L HCl 1.4 mmol/LBis(3-sulfopropyl)-disulfide, disodium salt 10 mg/L Polypropylene glycol80 mg/L Polyvinylpyrrolidone 10 mg/L pH 2.1

Plating was carried out at 25° C. and 1 A/dm² for 30 seconds.

Cross-sectional SEM observation is shown in FIG. 1. Void was not formedat anywhere including the side wall part of trenches.

Example 2

A silicon substrate having ULSI microwiring was subjected toelectrolytic copper plating using the following plating solution. Thesilicon substrate to be plated was the same as the one used in Example1, in which Cu seed layer had a thinnest thickness of 2 nm in trenches.

Composition of plating solution:

Copper (copper sulfate) 0.63 mol/L; Formic acid 1.0 mol/L HCl 1.4 mmol/LBis(3-sulfopropyl)-disulfide, disodium salt 10 mg/L Polypropylene glycol80 mg/L Polyvinylpyrrolidone 10 mg/L pH 1.9

Plating was carried out at 25° C. and 1 A/dm² for 30 seconds.

Cross-sectional SEM observation showed that void was not formed atanywhere including the side wall part of trenches.

Example 3

A silicon substrate having ULSI microwiring was subjected toelectrolytic copper plating using the following plating solution.

The silicon substrate to be plated was the same as the one used inExample 1 except that its Cu seed layer had a thinnest thickness of 1.8nm in trenches.

Composition of plating solution:

Copper (copper sulfate) 0.63 mol/L; Oxalic acid 0.1 mol/L HCl 1.4 mmol/LBis(3-sulfopropyl)-disulfide, disodium salt 10 mg/L Polypropylene glycol80 mg/L Polyvinylpyrrolidone 10 mg/L pH 2.5

Plating was carried out at 25° C. and 1 A/dm² for 30 seconds.

Cross-sectional SEM observation showed that void was not formed atanywhere including the side wall part of trenches.

Comparative example 1

Electrolytic copper plating was carried out in the same manner asExample 1 except that composition of the plating solution was changed asfollows.

Composition of plating solution:

Copper (copper sulfate) 0.63 mol/L Sulfuric acid 0.5 mol/L HCl 1.4mmol/L Bis(3-sulfopropyl)-disulfide, disodium salt 10 mg/L Polypropyleneglycol 80 mg/L Polyvinylpyrrolidone 10 mg/L <pH 1.0

Cross-sectional SEM observation is shown in FIG. 2. Voids (dark shadowyparts in circles) were observed in at least some of the side wall partof trenches.

1. A method for electrolytic copper plating for ULSI microwiringcomprising the step of forming damascene microwiring for ULSI with anelectrolytic copper plating aqueous solution for forming damascenemicrowiring for ULSI, characterized in that the plating solution has apH of from 1.8 to 3.0, contains copper sulfate at 0.05 to 1.5 mol/L andchloride ions at a concentration of 0.3 to 3.0 mmol/L, and comprises asaturated carboxylic acid at a concentration of from 0.01 to 2.0 mol/L.2. The method of claim 1, wherein the plating aqueous solution has a pHof 2.0 to 2.2.
 3. The method of claim 1, wherein the saturatedcarboxylic acid contains from 1 to 4 carbon atoms.
 4. The method ofclaim 3, wherein the saturated carboxylic acid is acetic acid.
 5. A ULSImicrowiring substrate in which the formation of voids on inner walls ofvias/trenches has been suppressed, characterized in that USLImicrowiring is formed by the method of claim 1.